U.S. Pat. No. 4,132,614 of Cuomo et al, "Etching by Sputtering from an Intermetallic Target to Form Negative Metallic Ions which Produce Etching of a Juxtaposed Surface" shows how to sputter negative ions of an intermetallic compound such as SmAu. U.S. Pat. No. 3,361,659 shows a sputtering chamber with a grid located between the target 18 and the substrate 24. The purposes for which the grid is used include (1) limiting deposition to negative ions only from the target; (2) providing a small electric field accelerating ionized gas atoms towards the substrate with a positive grid potential preventing incorporation of negative (OH) ions from contaminating the deposited film; (3) the grid can be within or outside the cathode dark space (i,e, near the target). U.S. Pat. No. 3,369,990 shows sputtering with oxygen to yield an oxide. U.S. Pat. No. 3,925,187 shows a system with a set of two simple one-metal targets which are commutable and no grid so that a sandwich of plural materials can be deposited without opening up the system, but the targets must be rotated and can be used only on a one-at-a-time basis; so graded, as distinguished from sharp variations in composition, cannot be achieved by cosputtering.
The formation of negative ions in the sputtering of highly ionic compounds such as TbF.sub.3 was reported by Hanak et al, "Effect of Secondary Electrons and Negative Ions on Sputtering of FIlms," J. Vac. Sci. Technol., Vol. 13, No. 1, January/February 1976, pp. 406-409. It describes cosputtering from a target of TbF.sub.3 and a target of ZnS, etching of the substrate directly opposite the TbF.sub.3 target. "It was postulated that etching of the substrate over the TbF.sub.3 was caused by the sputtering of the substrate by F.sup.- ions first released by the target and then accelerated and focused onto the substrate at full target-substrate voltage . . . ." It also speculates that other negative ions "such as those observed by Honig, [Refs.] 3,4 would be expected to produce a similar effect", referring to oxides, and ZnS, involving sulfide and oxide ion bombardment. Both examples selected were those of nonmetals. Halide ions are also mentioned. Honig is cited by Hanak as referring to Cl.sup.-, F.sup.-, Br.sup.-, O.sup.-, OH.sup.-, O.sub.2.sup.-, S.sup.-, C.sub.1.sup.- to C.sub.10.sup.-, SH.sup.-, Ag.sub.2.sup.- and Ag.sub.2 O.sup.- as sources of negative ions.